Estrogen deficiency caused by natural or surgical menopause yields changes that can have far reaching consequences on human health and well-being. Postmenopausal women show alterations in cognitive function that are thought to be attributable to estrogen loss. However, determining the relationship between estrogen states and cognitive function can be difficult to study in women for a variety of reasons. To circumvent these difficulties, the investigator has examined the effects of surgically-induced menopause in young monkeys. Loss of estrogen, independent of the effects of aging, can cause decrements in cognitive processing. However, because the average age at which women experience menopause is 50 years, the next step is to determine whether the interactive effects of age and estrogen loss are more deleterious to cognitive abilities than estrogen alone. The purpose of this project is to determine the relationship between estrogen states and cognitive function in middle-aged rhesus monkeys. Estrogen levels will be manipulated through ovariectomy and estrogen replacement therapy (ERT). This study will extend the initial behavioral investigations conducted in young female monkeys to middle- aged female monkeys and determine if the ovariectomized middle-aged female monkey is an appropriate animal model of the middle-aged postmenopausal woman.